Hard surface coatings are commonly deposited on cutting tools for metal machining, to provide a hard and wear resistant surface. But, coatings of this type may also be used to decrease wear between surfaces in a large variety of applications where two surfaces are in sliding contact, such as in bearings.
The present invention relates particularly to the art of coated cemented carbides and nitrides or similar hard materials such as cermets, ceramics and high speed steels. The method of depositing a thin refractory coating (1-20 .mu.m) of materials like alumina (Al.sub.2 O.sub.3), titanium carbide (TiC) and/or titanium nitride (TiN) onto, e.g., a cemented carbide cutting tool, is a well-established technology and the tool life of the coated cutting tool, when used in metal machining, is considerably prolonged. The prolonged services life of the tool may, under certain conditions, extend up to several 100 percent. Refractory coatings known in the art comprise either a single layer or a combination of multilayers. Modern commercial cutting tools are characterized by a plurality of layer combinations with double or multilayer structures. The total coating thickness varies between 1 and 20 .mu.m and the thickness of individual layers may be in the sub micrometer range (μm), i.e., the thickness of the individual layers varies between a few microns and a few tenths of a micron.
The established technologies for depositing such coatings are Physical (PVD) and Chemical (CVD) Vapor Deposition (see, e.g., U.S. Pat. Nos. 4,619,866 and 4,346,123). PVD coated commercial cutting tools of cemented carbides or high speed steels usually have a single coating of TiN, TiCN, or TiAlN, but combinations thereof also exist.
There exist several PVD techniques capable of producing refractory thin layers on cutting tools. The most established methods are ion plating, magnetron sputtering, arc discharge evaporation and IBAD (Ion Beam Assisted Deposition). Each method has its own merits and the intrinsic properties of the produced coating such as microstructure/grain size, hardness, state of stress, cohesion and adhesion to the underlying substrate may vary depending on the particular PVD method chosen. An improvement in the wear resistance or the edge integrity of a PVD coated cutting tool being used in a specific machining operation can thus be accomplished by optimizing one or several of the above-mentioned properties. Furthermore, new developments of the existing PVD techniques by, for instance, introducing unbalanced magnetrons in reactive sputtering (S. Kadlec, J. Musil and W.-D. Munz in J. Vac. Sci. Techn. A8(3), (1990), 1318) or applying a steered and/or filtered arc in cathodic arc deposition (H. Curtins in Surface and Coatings Technology, 76/77, (1995), 632 and K. Atari et al. in Surface and Coatings Technology, 43/44, (1990), 312) have resulted in a better control of the coating processes and a further improvement of the intrinsic properties of the coating material.
Conventional cutting tool materials like cemented carbides comprise at least one hard metallic compound and a binder, usually cobalt (Co), where the grain size of the hard compound, e.g., tungsten carbide (WC), is in the 1-5 .mu.m range. Recent developments have predicted improved tool properties in wear resistance, impact strength and hot hardness by applying tool materials based on ultrafine microstructures by using nanostructured WC—Co powders as raw materials (L. E. McClandlish, B. H. Kear and B. K. Kim, in NanoSTRUCTURED Materials, Vol. 1, pp. 119-124, 1992). Similar predictions have been made for ceramic tool materials by, for instance, applying silicon nitride/carbide-based (Si.sub.3 N.sub.4/SiC) nanocomposite ceramics and, for Al.sub.2 O.sub.3-based ceramics, equivalent nanocomposites based on alumina.
With nanocomposite nitride or carbide hard coating materials, it is understood a multilayered coating where the thickness of each individual nitride (or carbide) layer is in the nanometer range, 3-100 nm, or preferably 3-20 nm. Since a certain periodicity or repeat period of, e.g., a metal nitride layer sequence is invoked, these nanoscaled, multilayer coatings have been given the generic name of “superlattice” layers. A repeat period is the thickness of two adjacent metal nitride layers, i.e., with different metal elements in the sublayers. Several of the metal nitride superlattice coatings with the metal element selected from Ti, Nb, V and Ta, grown on both single- and polycrystalline substrates have shown an enhanced hardness for a particular repeat period, usually in the range of 3-10 nm.
Recent developments in the field, are found in WO9844163 and WO9848072 (U.S. Pat. No. 6,103,357), both by Sandvik AB, Sweden. In WO9844163 Hogmark et al disclose a coating of superlattice type, comprising a laminar, multilayered structure of refractory compounds in polycrystalline, repetitive form, (MX/NX) lambda/(MX/NX) lambda/(MX/NX) lambda/(MX/NX) lambda/ . . . where the alternating layers MX and NX are composed of metalnitrides or carbides with the metal element selected from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr and W. The repeat period lambda is essentially constant throughout the entire multilayered structure, and larger than 3 nm but smaller than 100 nm, preferably smaller than 25 nm. While Selinder et al in WO9848072 disclose a coating comprising of a laminar, multilayered structure of refractory compounds in polycrystalline, non-repetitive form, MX/NX/MX/NX/MX . . . , where the alternating layers MX and NX are composed of metal nitrides or carbides with the metal elements M and N are selected from the group consisting of Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, and W and mixtures therein, where in said coating the sequence of individual layer thicknesses has no repeat period but is essentially aperiodic throughout the entire mutlilayered structure, and where the said individual MX or NX layer thickness is larger than 0.1 nm but smaller than 30 nm and, varies essentially at random, and that the total thickness of said multilayered coating is larger than 0.5 .mu.m but smaller than 20 .mu.m.
In most prior art, two adjacent individual layers in the coating differ substantially in composition, whereby the deposition parameters in the PVD or CVD reactor must be altered in a correspondingly manner. The sequence of individual layer thicknesses can be fabricated by randomly opening and closing shutters from individual layer sources or by randomly switching such sources on and off. Another conceivable method is by randomly rotating or moving the to-be-coated tools, substrates, in front of said sources. Electron beam evaporation, magnetron sputtering or cathodic arc deposition or combinations thereof, are the preferred PVD methods for depositing nanostructured coatings.
Generally, the hardness and wear resistance of a material is related to the presence of glide planes in the material along which dislocations can propagate. Between coating layers of different constitution a change of material structure and glide plane position is likely to exist, promoting hardness and wear resistance of the material. A change of structure, from one phase to another, will result in a change in the glide plane symmetry and, hence, improved hardness and wear resistance.
U.S. Pat. No. 5,635,247 discloses a multilayer coating which is formed by depositing a kappa-alumina layer on a substrate, wet blasting the surface of the layer and then heat treating the coating to convert the kappa-alumina layer to the alfa-phase. One or more further alumina layers can then be deposited on the first layer. Thereby, layers of the same material but with different structures are created, resulting in broken or changed gliding planes in the border region between adjacent layers of different structure. Hence, an improved hardness and wear resistance is obtained.
However, the method described in the above document requires a special heat treatment of sub-layers in order to obtain the change of structure striven for. No prior multi-layer takes advantage of a multi-phase polytypic region, i.e a region where two or more of the lowest energy structures of a compound have the same, or almost the same, energies, for the compound or compounds used in the multi-layer in order to maximise the number of interfaces between sub-layers with different glide systems.